First you'll need to cut your leather into two rectangles about 1 inch oversize of the grip. (Always better to have too much than not enough.)
You want the grips undersized slightly on the outer dimensions (remember we're adding material). One at a time, sand off any finish from the wood, (this area should be rough to promote better adhesion with the two surfaces, 250 grit sandpaper works well for this,) and then paint on your wood glue...Yes, PAINT ON, you want a thin to medium coat that is even. Then you want to smooth out the leather over the outer most surface and any major contours such as a recessed thumb notch.

Done? Good, let them dry. Go have a beer or three.

Now you will need to start cutting to shape. Best to work the tougher areas first. Start stretching the leather around the contours and any areas that cause wrinkles you need to cut small angles (start small and work your way up till you get the leather to fit tightly). Using your knife cut the leather even with the flat of the grip so that the edges point toward the frame of the gun. Once you are done getting it cut to shape and glued down let it dry and check that all edges are secure. You can also go ahead and cut out the holes for the grip screws. I also painted on some glue around the edges to seal them and reduce fraying. Should end up looking like this:

Now once that part is complete, go have a beer.

Back? Good, now that the hard part is over. It's time for:

(WARNING: High quality leather dye will dye any porous surface known to mankind, PERMANENTLY, so be careful.)

Using your other brush...No you can't use the one covered in glue unless you managed to clean it completely. Glue and dye don't like each other very well. Moving on... We will now paint the dye onto the leather being sure to cover it completely and letting it soak in. You will probably have to go over it a few times. Once you reach your desired shade, set aside and let dry. You should now have something like this:

Congratulations, you're done. Unless you happen to have a clear coat for leather then you can paint that on to seal and shine them, (I don't).

I chose black because its what I had at hand and lets face it, you can't go wrong with black leather. Besides anything to piss off the PETA petters. Side note: I have still yet to see a single drop of red paint directed at me or any of my friends for wearing leather, dunno why not though.... Zinc and/or nickel plated steel studs are optional. Enjoy.

(FTC Complainer: No one paid me or gave me anything for this post so go take a long walk off a short pier into shark infested water.)